Figure 13.

The ψ of a tissue varies independently of the evaporation rate from that tissue. When the evaporation rates from the spongy mesophyll (red lines) and lower epidermis (blue lines) were modified by varying PPFD (A) or leaf airspace fraction (B) in the directions indicated by the gray arrows, while holding total evaporation rate constant by adjusting g_s, the ψ values of these tissues did not vary in the manner predicted by the hypothesis that an increase in evaporation rate from a tissue should cause its ψ to decline. In A, PPFD was varied between 0 and 1,500 μmol m⁻² s⁻¹ while holding all other parameters except g_s constant; g_s was adjusted between 0.4 and 0.24 mol m⁻² s⁻¹ in order to maintain a constant whole-leaf evaporation rate of 4.73 mmol m⁻² s⁻¹. In B, the mesophyll airspace fraction was adjusted between 10% and 63% (for spongy mesophyll) and between 6.7% and 40% (for palisade mesophyll) while holding all other parameters except g_s constant; g_s was adjusted between 0.4 and 0.409 mol m⁻² s⁻¹ to maintain whole-leaf evaporation rate constant at 7.07 mmol m⁻² s⁻¹. All anatomical parameters were set at their all-species average values (Supplemental Table S2).